1. Field of the Invention
The present invention relates to a combustion control apparatus and more particularly to a control apparatus for varying a combustion mode according to engine operating conditions.
2. Description of Background Arts
It is well known that lean air-fuel mixture combustion is an effective means for raising thermal efficiency of four cycle engines. The reason why lean mixture raises thermal efficiency is that it increases a ratio of specific heat capacities of working gas and provides an improved theoretical thermal efficiency. Also it is known that lean air-fuel mixture can reduce a pumping loss of an engine due to an increased intake air.
However, lean air-fuel mixture combustion provides an elongated combustion period of time and instable combustion and therefore lean air-fuel mixture has a lean limit. A stratified charge combustion strategy, in which a rich air-fuel mixture is concentrated about a spark plug by stratifying the air-fuel mixture gas in a cylinder by means of an in-cylinder fuel injection method, is effective for raising this lean limit. However, the stratified charge combustion strategy has an disadvantage that since the rich air-fuel mixture is concentrated about the spark plug, nitrogen oxides (NOx) emissions increase.
On the other hand, diesel engines have a high thermal efficiency due to the combustion by compression ignition and have a high lean limit. However, the diesel engines have low outputs and sometimes emit a large amount of smokes due to an inferior rate of air utilization at high loads.
To solve these problems, compression ignition gasoline engines in which air-gasoline mixture is ignited at multi-spots (multi-spots ignition) by an adiabatic compression, are proposed. The compression ignition gasoline engines have an advantage that since theses engines have short flame propagations due to multi-spots ignition, a fast burn is realized. As a result, local high temperature spots are difficult to be produced in a combustion chamber and therefore emissions of nitrogen oxides can be largely reduced.
In order to accomplish the compression ignition, the compression ratio must be established at a value as high as 15:1 to 18:1 of compression ratio. When an engine operates at high loads, the combustion pressure rapidly increases due to an increased amount of fuel injected and as a result knocking is apt to occur.
To solve this problem, Japanese Patent Application Laid-open No. Toku-Kai-Hei 9-287528 discloses a technique wherein the temperature of the combustion chamber is reduced by an external EGR effect or a technique wherein the temperature of intake air is regulated by a cooling device provided in an EGR passage.
The combustion control by the external EGR effect or the temperature of intake air has a dull response and therefore it is difficult to obtain a good follow-up ability to changing torques of the engine during traveling of the vehicle.
Further, in order to avoid a knocking phenomena of the engine, there is an idea of introducing a so-called xe2x80x9cmiller cyclexe2x80x9d in which an actual compression ratio is established to be smaller than an actual expansion ratio, however when the engine operates according to the miller cycle under a low speed and high load condition, an insufficient amount of air is supplied to the combustion chamber, this leading to a decrease in torque.
In an engine having a high compression ratio to realize a compression ignition, it is an object of the present invention to provide a combustion control apparatus for an engine capable of avoiding an occurrence of knocking and a decrease of torque in a high load area and reducing exhaust emissions in an overall operating area.
To achieve the object, a combustion control apparatus for an engine having a variable valve timing mechanism capable of varying valve timings of an exhaust valve and an intake valve, comprises a combustion mode establishing means for establishing a combustion mode to either of at least following two modes, a compression ignition combustion mode and a two-cycle spark ignition combustion mode according to an operational area of the engine and a valve timing establishing means for establishing said valve timings of the exhaust and intake valves so as to form a negative overlap period in which both exhaust and intake valves concurrently close in the neighborhood of the exhaust top dead center, when the combustion mode is established to the compression ignition combustion mode and for establishing the valve timings of the exhaust and intake valves so as to carry out a gas exchange once per one revolution of the engine, when the combustion mode is established to the two-cycle spark ignition combustion mode.